The present embodiments relate generally to systems and methods for measuring gas flux, and more particularly to systems and methods for measuring turbulent gas flux.
The increasing concentrations of carbon dioxide and other traces gases (e.g. H2O, CH4, N2O, NH3, etc.) in the atmosphere and the resulting greenhouse effect and climate change have become important topics for scientific research. In order to understand the global carbon balance, it is necessary to determine the rate at which carbon dioxide and energy exchanges between the atmosphere and terrestrial and oceanic ecosystems. The air within a few hundred meters above the earth's surface is mostly turbulent, so that turbulent structures (vortices of variable sizes) called “eddies” are responsible for the vertical transport of most of the gases, including carbon dioxide and water vapor, and also heat and momentum between the surface and the atmosphere. The rates of such transport can be calculated from simultaneous, high-frequency measurements of the vertical component of wind speed, the concentrations of carbon dioxide and water vapor, and the air temperature.
Currently, there are three main methods for computing turbulent gas flux rates, typically performed from towers, airplanes and other platforms: these are Eddy Covariance, Eddy Accumulation, and Relaxed Eddy Accumulation methods.
The Eddy Covariance (EC) method is the most direct and reliable method for gas flux measurements available to date. EC is a dominating method used in most turbulent flux measurements. EC is used as a standard for other turbulent flux measurement methods, and for any atmospheric flux measurement methods. However, EC requires high-speed gas concentration measurements (e.g., 5-10 Hz or more) in addition to the high-speed vertical wind speed measurements (e.g., 5-10 Hz or more). However, high-speed gas concentration measurement devices are expensive, and they do not exist for a number of gas species.
The Eddy Accumulation (EA) method is theoretically as reliable as EC, and it also requires high-speed vertical wind speed measurements, but it does not require high-speed gas concentration measurements. However, EA does need a highly sophisticated high-speed wind sampling system to distinguish updrafts from downdrafts, and another highly sophisticated high-speed system to sample gas into accumulation bags in proportion to the rates of the updrafts and downdrafts. Such systems performing at satisfactory levels are not presently available (see, e.g., Foken. T., 2009, Micrometeorology).
The Relaxed Eddy Accumulation (REA) method is a version of EA which does not require sampling in proportion to the rate of the updrafts and downdrafts. However, REA does also require a sophisticated high-speed wind sampling system to distinguish updrafts from downdrafts, while sampling into the accumulation bags is done at a constant flow rate. REA is not able to measure fluxes as reliably as EC or EA due to an empirical parameter required for calculations, but REA is used occasionally for measuring fluxes of gas species for which no high-speed gas measurement devices are available. Both the EA and the REA methods also have built-in measurement uncertainties associated with system configurations and components such as valve systems, sampling delays, tube time delays and attenuation, etc.
Therefore it is desirable to provide systems and methods that overcome the above and other problems.